Guido understands that the Tory Deputy Chief Whip John Randall has threatened to resign if Andrew Mitchell does not jump. He is currently not returning Guido’s calls, but a senior Tory confirms that there is serious consternation in the Whips office and has indicated that Randall is not the only one considering radical action. Guido speculated about this earlier and the threat is said to have “not fizzled out”.

A possible safe pair of hands if Mitchell does go, Randall was very close to former Chief Whip Patrick McLoughlin. Randall has form in walking on matters of principle: he resigned as an opposition whip in 2003. He is well regarded on the backbenches and Mitchell’s position would be untenable were he to go. Developing…

UPDATE: Looks like Tim Shipman has been doing a phone around:

John Randall told Tory MPs of his intention to quit in the Commons tea room last night before last division. But he's now gone to ground.